Efficient data storage in multi-plane memory devices

ABSTRACT

A method for data storage includes initially storing a sequence of data pages in a memory that includes multiple memory arrays, such that successive data pages in the sequence are stored in alternation in a first number of the memory arrays. The initially-stored data pages are rearranged in the memory so as to store the successive data pages in the sequence in a second number of the memory arrays, which is less than the first number. The rearranged data pages are read from the second number of the memory arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 61/014,048, filed Dec. 16, 2007, and U.S. Provisional PatentApplication 61/016,570, filed Dec. 25, 2007, whose disclosures areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to memory devices, andparticularly to methods and systems for data storage in memory deviceshaving multiple memory planes.

BACKGROUND OF THE INVENTION

Several types of memory devices, such as Flash memories, use arrays ofanalog memory cells for storing data. Each analog memory cell stores aquantity of an analog value, also referred to as a storage value, suchas an electrical charge or voltage. The storage value represents theinformation stored in the cell. In Flash memories, for example, eachanalog memory cell holds a certain amount of electrical charge. Therange of possible analog values is typically divided into regions, eachregion corresponding to one or more data bit values. Data is written toan analog memory cell by writing a nominal analog value that correspondsto the desired bit or bits.

Some memory devices, which are commonly referred to as Single-Level Cell(SLC) devices, store a single bit of information in each memory cell,i.e., each memory cell can be programmed to assume two possible memorystates. Higher-density devices, often referred to as Multi-Level Cell(MLC) devices, store two or more bits per memory cell, i.e., can beprogrammed to assume more than two possible memory states.

Flash memory devices are described, for example, by Bez et al., in“Introduction to Flash Memory,” Proceedings of the IEEE, volume 91,number 4, April, 2003, pages 489-502, which is incorporated herein byreference. Multi-level Flash cells and devices are described, forexample, by Eitan et al., in “Multilevel Flash Cells and theirTrade-Offs,” Proceedings of the 1996 IEEE International Electron DevicesMeeting (IEDM), New York, N.Y., pages 169-172, which is incorporatedherein by reference. The paper compares several kinds of multilevelFlash cells, such as common ground, DINOR, AND, NOR and NAND cells.

Eitan et al., describe another type of analog memory cell called NitrideRead Only Memory (NROM) in “Can NROM, a 2-bit, Trapping Storage NVMCell, Give a Real Challenge to Floating Gate Cells?” Proceedings of the1999 International Conference on Solid State Devices and Materials(SSDM), Tokyo, Japan, Sep. 21-24, 1999, pages 522-524, which isincorporated herein by reference. NROM cells are also described byMaayan et al., in “A 512 Mb NROM Flash Data Storage Memory with 8 MB/sData Rate”, Proceedings of the 2002 IEEE International Solid-StateCircuits Conference (ISSCC 2002), San Francisco, Calif., Feb. 3-7, 2002,pages 100-101, which is incorporated herein by reference. Otherexemplary types of analog memory cells are Floating Gate (FG) cells,Ferroelectric RAM (FRAM) cells, magnetic RAM (MRAM) cells, Charge TrapFlash (CTF) and phase change RAM (PRAM, also referred to as Phase ChangeMemory—PCM) cells. FRAM, MRAM and PRAM cells are described, for example,by Kim and Koh in “Future Memory Technology including Emerging NewMemories,” Proceedings of the 24^(th) International Conference onMicroelectronics (MIEL), Nis, Serbia and Montenegro, May 16-19, 2004,volume 1, pages 377-384, which is incorporated herein by reference.

Some memory device configurations comprise multiple memory arrays, oftenreferred to as memory planes. Various methods and systems for storingdata in multi-plane memory devices are known in the art. For example,U.S. Pat. No. 7,280,398, whose disclosure is incorporated herein byreference, describes a system and method for performing memoryoperations in a multi-plane Flash memory. Commands and addresses aresequentially provided to the memory for memory operations in memoryplanes. The memory operations are initiated sequentially, and the memoryoperation for at least one of the memory planes is initiated during thememory operation for another memory plane.

U.S. Pat. No. 7,257,027, whose disclosure is incorporated herein byreference, describes a NAND-type Flash memory device having amulti-plane structure. Page buffers are divided into even page buffersand odd page buffers and are driven at the same time. Cells connected toeven bit lines within one page and cell connected to odd bit lineswithin one page are programmed, read and copyback-programmed at the sametime.

U.S. Application Publication 2006/0218359, whose disclosure isincorporated herein by reference, describes a computational systemcomprising a controller and a multi-plane solid state memory deviceaccessible to the controller. The controller is configured to provideaccess to a virtual block having a virtual block address that representsa first block from a first plane of the multi-plane solid state memorydevice and represents a second block from a second plane of themulti-plane solid state memory device.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method for datastorage, including:

initially storing a sequence of data pages in a memory that includesmultiple memory arrays, such that successive data pages in the sequenceare stored in alternation in a first number of the memory arrays;

rearranging the initially-stored data pages in the memory so as to storethe successive data pages in the sequence in a second number of thememory arrays, which is less than the first number; and

reading the rearranged data pages from the second number of the memoryarrays.

In some embodiments, reading the rearranged data pages includes readinga given data page and one or more neighboring data pages from the secondnumber of the memory arrays and canceling cross-coupling interference inthe given data page responsively to the read neighboring data pages. Ina disclosed embodiment, reading the rearranged data pages includescaching the read data pages in a cache memory such that both the givendata page and the neighboring data pages are cached in the cache memoryat a given time, and canceling the cross-coupling interference includescanceling the interference using the cached given data page andneighboring data pages.

In another embodiment, the second number of the memory arrays includes asingle memory array. In yet another embodiment, the second number of thememory arrays includes two or more memory arrays, and rearranging theinitially-stored data pages includes storing the successive data pagesin alternation in the second number of the memory arrays.

In still another embodiment, rearranging the initially-stored data pagesincludes rearranging the initially-stored data pages so as to store thesuccessive data pages in a third number of the memory arrays, which issmaller than the first number and larger than the second number, andrearranging the data pages stored in the third number of the memoryarrays so as to store the successive data pages in the second number ofthe memory arrays.

In an embodiment, rearranging the initially-stored data pages includesdefining multiple groups of the memory arrays, each group having thesecond number of the memory arrays, and storing respective subsets ofthe successive data pages in the groups. Storing the subsets in thegroups may include sending multi-plane write commands, each commandspecifying storage of the second number of the data pages in each of thegroups. In a disclosed embodiment, initially storing the sequenceincludes sending multi-plane write commands, each command specifyingstorage of the first number of the data pages in the first number of thememory arrays.

Initially storing the sequence may include sending an acknowledgementindication responsively to a successful completion of initially-storingthe sequence, irrespective of rearrangement of the initially-stored datapages. In some embodiments, the initially-stored data pages arerearranged during an idle period in which no memory access operationsare performed.

In some embodiments, each of the memory arrays includes a plurality ofanalog memory cells, initially storing the sequence includes programmingthe memory cells in the first number of the memory arrays using firstprogramming parameters, and rearranging the data pages includesprogramming the memory cells in the second number of the memory arraysusing second programming parameters, different from the firstprogramming parameters. In a disclosed embodiment, programming thememory cells using the first programming parameters includes programmingthe memory cells to assume a first number of programming levels, andprogramming the memory cells using the second programming parametersincludes programming the memory cells to assume a second number ofprogramming levels, larger than the first number.

Additionally or alternatively, programming the memory cells using thefirst programming parameters includes applying to the memory cells afirst sequence of programming pulses whose amplitude increasesincrementally by a first step size, and programming the memory cellsusing the second programming parameters includes applying to the memorycells a second sequence of programming pulses whose amplitude increasesincrementally by a second step size, smaller than the first step size.

In another embodiment, reading the rearranged data pages includes:

receiving a request from a host to retrieve one or more requested datapages in the sequence;

responsively to the request, reading from the second number of thememory arrays one or more non-requested data pages in the sequence,which were not requested by the host, as well as the requested datapages;

caching the read requested and non-requested data pages; and

canceling cross-coupling interference in at least one of the requesteddata pages using the cached requested and non-requested data pages.

In yet another embodiment, initially storing the data pages includesstoring the data pages in a Dynamic Random Access Memory (DRAM), andrearranging the initially-stored data pages includes copying the datapages from the DRAM to a non-volatile memory.

In still another embodiment, the memory cells in the second number ofthe memory arrays are arranged along multiple word lines, andrearranging the initially-stored data pages includes storing the datapages in an ascending order of the word lines and in a reverse order ofthe sequence.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for data storage, including:

storing a sequence of data pages in a memory;

receiving a series of requests from a host to sequentially retrieve thestored data pages from the memory;

responsively to a given request received at a given time, reading fromthe memory one or more non-requested data pages in the sequence, whichwere not yet requested by the host at the given time, as well as readingone or more requested data pages specified in the given request;

caching the read requested and non-requested data pages; and

canceling cross-coupling interference in at least one of the requesteddata pages using the cached requested and non-requested data pages.

There is also provided, in accordance with an embodiment of the presentinvention, a data storage apparatus, including:

a memory, which includes multiple memory arrays; and

circuitry, which is coupled to initially store a sequence of data pagesin the memory such that successive data pages in the sequence are storedin alternation in a first number of the memory arrays, to rearrange theinitially-stored data pages in the memory so as to store the successivedata pages in the sequence in a second number of the memory arrays,which is less than the first number, and to read the rearranged datapages from the second number of the memory arrays.

There is further provided, in accordance with an embodiment of thepresent invention, a data storage apparatus, including:

a memory; and

a processor, which is coupled to store a sequence of data pages in amemory, to receive a series of requests from a host to sequentiallyretrieve the stored data pages from the memory, to read from the memory,responsively to a given request received at a given time, one or morenon-requested data pages in the sequence that were not yet requested bythe host at the given time, as well as reading one or more requesteddata pages specified in the given request, to cache the read requestedand non-requested data pages, and to cancel cross-coupling interferencein at least one of the requested data pages using the cached requestedand non-requested data pages.

There is additionally provided, in accordance with an embodiment of thepresent invention, a data storage apparatus, including:

an interface, which is operative to communicate with a memory thatincludes multiple memory arrays; and

a processor, which is coupled to initially store a sequence of datapages in the memory such that successive data pages in the sequence arestored in alternation in a first number of the memory arrays, to causethe memory to rearrange the initially-stored data pages in the memory soas to store the successive data pages in the sequence in a second numberof the memory arrays, which is less than the first number, and to readthe rearranged data pages from the second number of the memory arrays.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a memorysystem, in accordance with an embodiment of the present invention;

FIG. 2 is a diagram that schematically illustrates a process forrearranging data from a multi-plane format to a single-plane format, inaccordance with an embodiment of the present invention;

FIG. 3 is a flow chart that schematically illustrates a method for datastorage and retrieval, in accordance with an embodiment of the presentinvention;

FIG. 4 is a diagram that schematically illustrates a process forrearranging data between two multi-plane formats, in accordance with anembodiment of the present invention; and

FIG. 5 is a block diagram that schematically illustrates a process forretrieving data from a memory device, in accordance with an embodimentof the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Some memory devices comprise multiple separate memory arrays that areoperated in parallel in order to increase the speed of memory accessoperations. The separate arrays are commonly referred to as memoryplanes, and such memory devices are referred to as multi-plane devices.Multi-plane memory devices usually support multi-plane read and writecommands, which perform reading and writing of multiple pages tomultiple memory planes in parallel.

On the other hand, in some applications that involve caching of readpages, there is a distinct disadvantage in storing the data in multiplememory planes. The disadvantage stems from the fact that retrievedmemory pages are usually cached in the order in which they are read fromthe memory device. When the read data is stored in multiple planes,pages that originate from different planes are cached in an alternatingorder. Since the memory device is usually capable of caching only alimited number of pages, the number of cached pages belonging to a givenmemory plane is small.

Having access to only a small number of pages from the same plane can beproblematic for some applications. For example, a given memory page maysuffer from cross-coupling interference from neighboring pages in thesame memory plane. The cross-coupling interference can be canceled usingthe data read from the interfered page together with the data read fromits neighbor pages. For this purpose, a page buffer used forinterference cancellation should be dimensioned so that both theinterfered memory page and its neighbors are present in the buffersimultaneously. When the number of planes is high, and/or when a givenpage is subject to interference from a large number of pages, therequired buffer size may become prohibitive.

Embodiments of the present invention that are described hereinbelowprovide improved methods and systems for data storage in multi-planememory devices. The disclosed methods and systems initially store asequence of data pages in a multi-plane manner, such that successivepages in the sequence are stored in a certain number of memory planes inalternation. At a later time, the data is rearranged in the memory so asto store successive data pages in a smaller number of memory planes. Insome embodiments, the data is rearranged so as to store it sequentiallyin a single memory plane.

When the data is to be retrieved, the data is read from the smallernumber of memory planes into which it was rearranged. Reading the datafrom a small number of memory planes (e.g., from a single plane)simplifies the task of canceling cross-coupling interference, andenables the use of a smaller page buffer. Since interferencecancellation is performed using the cached data and does not requireadditional dedicated read operations, the overall reading speed isincreased, as well.

Thus, the methods and systems described herein enable fast and efficientdata programming using the powerful features of multi-plane storage. Byrearranging the stored data into a smaller number of memory planes, thedisclosed methods and systems provide high-performance reading andcross-coupling interference cancellation using limited-size buffermemories.

Several examples of multi-plane to single-plane and multi-plane tomulti-plane rearrangement processes are described herein. In someembodiments, the data is rearranged by applying a series of progressiverearrangement operations, each operation reducing the number of planesover which successive pages are cached. Some disclosed embodimentsemploy a read-ahead mechanism, in which data pages that were not yetrequested (e.g., by a host) are read, buffered and used for cancelingcross-coupling interference in requested pages.

System Description

FIG. 1 is a block diagram that schematically illustrates a memory system20, in accordance with an embodiment of the present invention. System 20can be used in various host systems and devices, such as in computingdevices, cellular phones or other communication terminals, removablememory modules (“disk-on-key” devices), Solid State Disks (SSD), digitalcameras, music and other media players and/or any other system or devicein which data is stored and retrieved.

System 20 comprises a memory device 24, which stores data in multipleanalog memory cells 32, which are arranged in two or more separate cellarrays 30. Cell arrays 30 are also referred to as memory planes, anddevice 24 is thus referred to as a multi-plane device. In someembodiments, the memory device may comprise multiple separatesemiconductor dies 28, each comprising one or more of the memory planes.For example, device 24 may comprise a Multi-Chip Package (MCP) in whichthe multiple dies are assembled. In the example embodiment of FIG. 1,memory device 24 comprises four dies 28, each comprising two memoryplanes 30, to form a total of eight planes in the device. In alternativeembodiments, however any other suitable numbers of memory planes, diesand planes per die can also be used.

In the context of the present patent application and in the claims, theterm “analog memory cell” is used to describe any memory cell that holdsa continuous, analog value of a physical parameter, such as anelectrical voltage or charge. Arrays 30 may comprise analog memory cellsof any kind, such as, for example, NAND, NOR and CTF Flash cells, PCM,NROM, FRAM, MRAM and DRAM cells.

The charge levels stored in the cells and/or the analog voltages orcurrents written into and read out of the cells are referred to hereincollectively as analog values or storage values. Although theembodiments described herein mainly address threshold voltages, themethods and systems described herein may be used with any other suitablekind of storage values.

System 20 stores data in the analog memory cells by programming thecells of each memory plane to assume respective memory states, which arealso referred to as programming levels. The programming levels areselected from a finite set of possible levels, and each levelcorresponds to a certain nominal storage value. For example, a 2bit/cell MLC can be programmed to assume one of four possibleprogramming levels by writing one of four possible nominal storagevalues to the cell.

Memory device 24 comprises a reading/writing (R/W) unit 36, whichconverts data for storage in the memory device to storage values andwrites them into memory cells 32 of the different memory planes. Inalternative embodiments, the R/W unit does not perform the conversion,but is provided with voltage samples, i.e., with the storage values forstorage in the cells. The R/W unit typically (although not necessarily)programs the cells using an iterative Program and Verify (P&V) process,as is known in the art. When reading data out of memory cells 32, R/Wunit 36 converts the storage values of cells 32 into digital sampleshaving a resolution of one or more bits. Data is typically written toand read from the memory cells of a given plane in groups that arereferred to as pages.

The storage and retrieval of data in and out of memory device 24 isperformed by a Memory Signal Processor (MSP) 40. MSP 40 comprises aninterface 44 for communicating with memory device 24, and a signalprocessing unit 48, which processes the data that is written into andread from device 24. In some embodiments, unit 48 produces the storagevalues for storing in the memory cells and provides these values to R/Wunit 36. Alternatively, unit 48 provides the data for storage, and theconversion to storage values is carried out by the R/W unit internallyto the memory device.

MSP 40 comprises a page buffer 38, for temporarily caching multiple datapages that are processed by the MSP. For example, when cancelingcross-coupling interference in a given page, the MSP typically uses theread results of neighboring pages that are cached in the page buffer.Buffer 38 typically comprises Random Access Memory (RAM) cells. In atypical application, the size of buffer 38 is on the order of ten pages,although any other suitable size can also be used. The use of pagebuffer 38 in performing data storage and retrieval is addressed ingreater detail below.

MSP 40 communicates with a host 52, for accepting data for storage inthe memory device and for outputting data retrieved from the memorydevice. MSP 40, and in particular unit 48, may be implemented inhardware. Alternatively, MSP 40 may comprise a microprocessor that runssuitable software, or a combination of hardware and software elements.

The configuration of FIG. 1 is an exemplary system configuration, whichis shown purely for the sake of conceptual clarity. Any other suitablememory system configuration can also be used. Elements that are notnecessary for understanding the principles of the present invention,such as various interfaces, addressing circuits, timing and sequencingcircuits and debugging circuits, have been omitted from the figure forclarity.

In the exemplary system configuration shown in FIG. 1, memory device 24and MSP 40 are implemented as two separate Integrated Circuits (ICs). Inalternative embodiments, however, the memory device and MSP may beintegrated on separate semiconductor dies in a single Multi-Chip Package(MCP) or System on Chip (SoC). Further alternatively, some or all of theMSP circuitry may reside on the same die on which elements of the memorydevice (e.g., the R/W unit, the page cache and/or one or more of thememory planes) are disposed. Further alternatively, some or all of thefunctionality of MSP 40 can be implemented in software and carried outby a processor or other element of the host system, or by a suitablememory controller. In some implementations, a single MSP 40 may beconnected to multiple memory devices 24. In yet another embodiment, someor all of the MSP functionality may be carried out by a separate unit,referred to as a memory extension, which acts as a slave of memorydevice 24.

Typically, the MSP (or any other controller or processor that carriesout some or all of the methods described herein) comprises ageneral-purpose processor, which is programmed in software to carry outthe functions described herein. The software may be downloaded to theprocessor in electronic form, over a network, for example, or it may,alternatively or additionally, be provided and/or stored on tangiblemedia, such as magnetic, optical, or electronic memory.

Within a given memory plane, memory cells 32 are typically arranged in agrid having multiple rows and columns, commonly referred to as wordlines and bit lines, respectively. The memory plane is typically dividedinto multiple pages, i.e., groups of memory cells that are programmedand read simultaneously. Cells are typically erased in groups of wordlines that are referred to as erasure blocks.

In some embodiments, R/W unit 36 programs memory cells 32 using aniterative Program and Verify (P&V) process. In a typical P&V process, anentire memory page is written by applying a sequence of programmingpulses to a group of memory cells that are mapped to this page. Thelevel of the programming pulses increases incrementally from pulse topulse. The storage values programmed in the cells are read (“verified”)after each pulse, and the iterations continue until the desired levelsare reached.

In FIG. 1, R/W unit 36 is shown as a single unit that performs readingand writing in the different memory planes. This configuration is shownpurely for the sake of clarity. In a typical implementation, each memoryplane (or each die) comprise some on-chip R/W circuitry for performingreading and writing in the memory cells of the respective plane or die.Additionally or alternatively, some R/W functions may be performed bycircuitry that is common to different planes, e.g., to the entiredevice. Thus, the functions of R/W unit 36 may be partitioned in anysuitable manner among on-chip circuitry disposed on the different diesand circuitry that is common to multiple different dies.

Multi-Plane VS Single-Plane Storage and Retrieval

In some embodiments, multi-plane device 24 supports multi-plane writecommands, which write data into multiple pages belonging to multipledifferent planes in parallel. In the embodiment of FIG. 1, device 24supports multi-plane commands that write eight data pages into the eightrespective memory planes of the device concurrently. Multi-plane writecommands increase the writing speed of the device, since they enable thedevice to perform multiple memory access operations in parallel, or atleast in semi-parallel. Device 24 may also support multi-plane readcommands that read data from multiple pages belonging to differentplanes in parallel.

In some practical cases, however, there is a distinct disadvantage inholding data in device 24 (and particularly in reading data from thedevice) in a multi-plane manner. The disadvantage stems from the factthat the MSP typically caches retrieved pages in page buffer 38 in theorder in which they are read from the device. When the data is storedand read in a multi-plane manner, buffer 38 holds pages that alternateamong the different planes. In some applications (e.g., when performingcross-coupling interference cancellation), it is desirable to cache acertain minimum number of successive pages from the same memory plane ata given time. When the data is stored and read in a multi-plane manner,the size of page buffer 38 has to be increased in order to meet theabove-mentioned requirement. On the other hand, when the data is storedand read in a single-plane manner, a modest-size buffer is usuallysufficient for caching the desired number of pages from a given plane.

The description that follows demonstrates the advantage of storing andreading data in a single-plane manner in canceling cross-couplinginterference. The storage values stored in cells 32 are sometimesdistorted by cross-coupling interference from other memory cells. Theinterference in a certain interfered page is typically caused byneighboring pages in the same memory plane as the interfered page. Insome embodiments, MSP 40 cancels the cross-coupling interference in agiven interfered page using the storage values of the cells in theinterfering neighboring pages. The data read from the interfered page,as well as the data read from the neighboring pages, is cached in pagebuffer 38.

When data is stored and retrieved from the device in a multi-planemanner, buffer 38 will hold alternating pages that originate from thedifferent planes. Most of these cached pages are irrelevant forcanceling the interference in the interfered page, since they originatefrom other planes and are certainly not neighbors of this page. In orderto perform interference cancellation in a given interfered page, thesize of buffer 38 is typically defined so that both the interfered pageand its neighbors are likely to be cached in the page buffer at the sametime. Otherwise, one or more of the interfering pages may need to beread separately for the sake of interference cancellation, a task thatcomplicates the reading operation and reduces reading speed.

When the pages are cached in alternating order among the differentplanes (because the data was read in a multi-plane manner), the desiredsize of buffer 38 increases considerably. The effect of multi-planestorage on the size of buffer 38 is especially severe when theinterference in a given page is caused by a large number of neighboringpages, for example when cells 32 comprise multi-level cells such thateach cell stores data pertaining to multiple pages.

In summary, multi-plane storage is typically advantageous in improvingprogramming speed. Single-plane storage, on the other hand, is typicallyadvantageous in improving reading time when performing interferencecancellation using a limited-size page buffer.

Data Rearrangement from Multi-Plane to Single-Plane Format

Embodiments of the present invention that are described herein provideimproved methods and systems for data storage in multi-plane memorydevices. The disclosed methods and systems initially store data in thememory device in a multi-plane manner, such that the data is stored inalternation in a certain number of memory planes. At a later time, thedata is rearranged, so as to store the data in alternation in a smallernumber of memory planes. In some embodiments, the data is rearranged soas to store it sequentially in a single memory plane.

When the data is to be retrieved, the data is read from the smallernumber of memory planes into which it was rearranged. Reading the datafrom a small number of memory planes (e.g., a single plane) simplifiesthe task of canceling cross-coupling interference, and enables the useof a smaller page buffer. The efficient use of the page buffer enablesusing a given-size buffer for additional purposes. For example, the MSPcan read soft information from the data pages, e.g., by reading thecells using multiple read thresholds, and cache the read results at ahigher resolution.

In the embodiments described herein, the various functions describedherein (e.g., initial storage, data rearrangement, subsequent dataretrieval and interference cancellation) are carried out by MSP 40 (orother controller that controls device 24). Alternatively, however, thesefunctions can be carried out by elements internal to device 24 (e.g., byunit 36) or by elements external to the device (e.g., MSP 40). In someimplementations, page buffer 38 may be located internally to the memorydevice. In the context of the present patent application and in theclaims, these elements are referred to collectively as data storagecircuitry, which is coupled to the memory and carries out the methodsdescribed herein. The disclosed methods can be partitioned among thedifferent elements in any suitable way.

Multi-Plane to Single-Plane Rearrangement Example

FIG. 2 is a diagram that schematically illustrates a process forrearranging data from a multi-plane format to a single-plane format, inaccordance with an embodiment of the present invention. In the exampleof FIG. 2, MSP 40 stores a sequence of data pages denoted P0, P1, P2,P3, . . . in memory device 24. For example, the sequence may comprisedata of a large file that is to be stored sequentially in the memorydevice.

The MSP initially stores the data pages in four memory planes 60A . . .60D, such that successive data pages in the sequence are stored in thedifferent memory planes in alternation. In some embodiments, the MSPstores the data by sending multi-plane write commands to the memorydevice. In the present example, the first multi-plane write commandstores pages P0 . . . P3 in planes 60A . . . 60D, respectively. The nextmulti-plane write command stores pages P4 . . . P7 in planes 60A . . .60D, respectively, and so on.

At a later time, the MSP rearranges the initially-stored data pages intoa single plane 64A. As can be seen in the figure, after rearrangementthe data pages in the sequence are stored sequentially in a singlememory plane. The MSP may rearrange the data at any suitable time afterinitial storage of the data, such as immediately following the initialmulti-plane storage, or during an idle period in which the MSP andmemory device are not busy performing other storage tasks.

When the MSP receives a request to retrieve the data, it reads therearranged data pages from memory plane 64A, typically usingsingle-plane read commands. The read pages are cached in page buffer 38in the order in which they are read. In particular, neighboring pages inmemory plane 64A will be cached adjacently to one another in the pagebuffer. MSP 40 may perform interference cancellation on a giveninterfered page by reading both the interfered page and its neighborsfrom buffer 38.

Thus, the disclosed method provides fast initial data storage usingmulti-plane write commands, without compromising the ability to performinterference cancellation using a limited-size buffer memory.

In the description above, data rearrangement is initiated and performedby the MSP. Alternatively, data rearrangement can be performed bycircuitry that is internal to the memory device, e.g., by the R/W unit.Data rearrangement can be carried out using any suitable combination ofsingle- or multi-plane read and write commands, and/or using internalsingle- or multi-plane copy-back commands in the memory device.

When using copy-back commands, errors in re-reading the initially-storeddata may introduce errors into the rearranged data. (When the data isrearranged using separate read and write commands, the read datatypically undergoes Error Correction Code (ECC) decoding, which reducesthe likelihood of errors. Rearrangement using copy-back commands usuallydoes not include ECC decoding.) The MSP may reduce the likelihood ofsuch errors using several means. For example, the MSP may perform thecopy-back operation shortly after the data was initially stored, so thatvoltage drifts and other impairments that increase over time will not bedominant. The MSP may allow a certain number of errors, which willeventually be corrected when the rearranged data is read. Additionallyor alternatively, the MSP may retrieve the data read by the copy-backoperation, perform ECC decoding, and send the corrected bits back to thememory device.

In some embodiments, the data is rearranged by copying the data todifferent memory locations, e.g., to another memory block.Alternatively, data rearrangement can be performed in-place, so that therearranged data occupies the same (or at least partially overlapping)memory locations as the initially-stored data.

Data Storage Method Description

FIG. 3 is a flow chart that schematically illustrates a method for datastorage and retrieval, in accordance with an embodiment of the presentinvention. The method begins with MSP 40 accepting from host 52 data forstorage, at an input step 70. The MSP formats the data in a sequence ofdata pages, and stores the data pages in multiple memory planes inalternation, at an initial storage step 74. Typically, the MSP storesthe data by issuing fast multi-plane write commands to the memorydevice.

In some embodiments, once the initial storage is completed successfully,the MSP sends to the host an acknowledgement indicating that the datahas been stored. Thus, the host application enjoys the fast storage timeenabled by the multi-plane storage, irrespective of the subsequentrearrangement operations performed on the data.

At a later time after initial storage of the data, the MSP rearrangesthe data pages into a single-plane format, at a single-planerearrangement step 78. The MSP copies the initially-stored data pagesinto a single memory plane, such that successive pages are storedsequentially in this plane. Typically, the MSP erases theinitially-stored data after successfully completing the rearrangement.

At a certain time after the data has been rearranged, the MSP receives arequest from host 52 to retrieve the data, at a request acceptance step82. In response to the request, the MSP reads the data pages from thesingle memory plane into which they have been copied, at a single-planereading step 86.

MSP 40 caches the read data pages in memory buffer 38, at a caching step90. Buffer 38 has a finite size of N pages. Thus, cache 38 is able tohold the N most-recently read pages.

When a given page is distorted by cross-coupling interference, MSP 40applies interference cancellation to this page using the data read fromneighboring pages, which are cached in buffer 38, at an interferencecancellation step 94. Various interference cancellation schemes areknown in the art, and any suitable scheme can be used by MSP 40. Severalexample techniques for canceling cross-coupling interference aredescribed in PCT International Publications WO 2007/132453, WO2007/132457, WO 2008/053472 and WO 2008/111058, whose disclosures areincorporated herein by reference. In some embodiments, the data storedin each page is encoded with an ECC, and the MSP applies interferencecancellation in a given page in response to a failure to decode the ECC.

Since in the single-plane format successive data pages in the sequenceare stored sequentially in the memory plane, both the interfered pageand its neighbors will be cached in buffer 38 at the same time. Thus,the MSP is able to apply interference cancellation using the cacheddata, without a need for additional read operations. Followinginterference cancellation, the MSP reconstructs and outputs therequested data to the host, at an output step 96.

Multi-Plane to Multi-Plane Data Rearrangement

The multi-plane to single-plane rearrangement process described in FIGS.2 and 3 above involves a number of read and write operations, which takea certain time to complete. Typically, most of this time is taken bysingle-plane write operations. As such, it is possible that the hostwill request retrieval of the data before the rearrangement process iscompleted. In some embodiments, such a scenario (as well as any otherscenario in which the data rearrangement process failed to complete) canbe resolved by retrieving the initially-stored pages, in the multi-planeformat. In these embodiments, data retrieval may be slower, especiallywhen interference cancellation is performed. Still, the stored data isnot lost and can be provided to the host.

Alternatively, the likelihood of encountering such a scenario can bereduced by reducing the data rearrangement time. In some embodiments,instead of rearranging the initially-stored data into a single-planeformat, the MSP rearranges the data into another multi-plane formathaving a smaller number of pages. For example, the MSP can initiallystore the data pages in eight memory planes, and later rearrange thedata by storing it in four memory planes. Rearranging data from eightplanes to four planes takes a considerably shorter time than rearrangingthe data from eight planes to a single plane. Therefore, the host isless likely to request the data before rearrangement is complete. On theother hand, the achievable reading speed, assuming interferencecancellation that uses a limited-size page buffer, is reduced.

Thus, conversion to a multi-plane format having a smaller number ofplanes provides a trade-off between conversion speed and readingperformance. Such a trade-off may be preferable in some practical cases.

FIG. 4 is a diagram that schematically illustrates a process forrearranging data between two multi-plane formats, in accordance with anembodiment of the present invention. In the example of FIG. 4, the MSPinitially stores a sequence of data pages in four memory planes 60A . .. 60D, similarly to the initial storage process of FIG. 2 above.

At a later time (e.g., during an idle period of the system), the MSPrearranges the data pages into a two-plane format. Memory planes 64A . .. 64D are divided into two groups 98A and 98B, each comprising twomemory planes. The MSP rearranges the sequence of pages such thatsuccessive pages in the sequence are stored in alternation in two memoryplanes. As can be seen in the figure, pages P0 . . . P7 are stored inmemory planes 64A and 64B. Pages P8 . . . P15 are stored in memoryplanes 64C and 64D.

When retrieving the data sequentially, the sequence of pages can be readusing multi-plane (e.g., two-plane or four-plane) read commands. In thepresent example, a first two-plane read command retrieves pages P0 andP1, a second command retrieves pages P2 and P3, and so on. When the datapages stored in group 98A are retrieved, another sequence of two-planeread commands can read pages P8 and P9, P10 and P11, P12 and P13, andfinally P14 and P15.

Note that the data rearrangement process can be carried out usingfour-plane read and write commands, even though the resultingarrangement is a two-plane arrangement. Each multi-plane command storestwo data pages in each of groups 98A and 98B. In these embodiments, theR/W unit reads and caches the initially-stored pages P0 . . . P15 fromplanes 60A . . . 60D using four-plane read commands. Then, the R/W unitstores the cached pages in planes 64A . . . 64D at the appropriate order(e.g., store pages P0, P1, P8, P9 in a given command, store pages P2,P3, P10, P11 in another command, etc.)

In some embodiments, the MSP performs multi-plane to single-planerearrangement (or multi-plane to multi-plane rearrangement) by applyinga series of progressive rearrangement operations, such that eachrearrangement operation reduces the number of planes over whichsuccessive pages are stored. For example, the MSP may rearrange the datafrom four-plane to two-plane format, as shown in FIG. 4. Then, the MSPcan rearrange each of groups 98A and 98B from two-plane intosingle-level format, as shown in FIGS. 2 and 3 above. Any desired numberof intermediate rearrangement steps can be used.

This progressive rearrangement process provides a soft trade-off betweenrearrangement speed and reading performance. When using this process,the memory device can provide data formats, whose reading performanceincreases over time. If the host requests the data when therearrangement process is still in progress, the best available dataformat (the completed format having the smallest number of planes) canbe used for reading.

In some embodiments, the initial storage operation (e.g., step 74 inFIG. 3 above) may use a programming configuration that is different fromthe programming configuration used for data rearrangement (e.g., at step78 of FIG. 3). Typically, the initial storage operation is temporary bynature, and is shortly followed by data rearrangement. As such, theinitial storage operation can be optimized for speed, possibly at theexpense of endurance and storage density. The programming configurationused for storing the rearranged data, on the other hand, is typicallyoptimized for endurance and density.

Various programming parameters may differ between the initial storageoperation and the rearrangement operation. For example, the programmingspeed of the initially-stored data can be increased by using arelatively small number of programming levels (e.g., using only twolevels), so that each cell stores a small number of bits. The rearrangeddata, on the other hand, may be subsequently stored using a highernumber of programming levels so as to achieve higher density.

As another example, when the cells are programmed using an iterativeProgram and Verify (P&V) process, the initial storage operation may usea relatively large P&V step size (i.e., a relatively large incrementbetween successive programming pulses), so as to increase programmingspeed. The rearrangement operation, on the other hand, is typicallyperformed using a smaller P&V step size, so as to improve programmingaccuracy and thus the reliability and endurance of the stored data.Additionally or alternatively, the MSP (using R/W unit 36) may vary anyother suitable programming parameter between the initial storageoperation and the storage operations used in data rearrangement.

Cross-Coupling Cancellation with Page Read-Ahead

As noted above, MSP 40 cancels cross-coupling interference in a giveinterfered page based on the data of neighboring pages. In someembodiments, the MSP reads pages that were not yet requested by thehost, and uses these pages for canceling interference in requestedpages. This function is referred to herein as read-ahead. In many cases,especially when reading large amounts of data stored sequentially insuccessive pages, the read-ahead pages are likely to be requested by thehost while they are still cached by the MSP, so that the read operationsof these pages are not redundant.

FIG. 5 is a block diagram that schematically illustrates a process forretrieving data from memory device 24 by MSP 40, in accordance with anembodiment of the present invention. In this embodiment, page buffer 38is capable of caching N memory pages. The host instructs the MSP to readdata pages sequentially from the memory device, such as by issuing aseries of read requests, each specifying one or more data pages that areto be read.

The MSP reads N pages in advance (i.e., before these pages are actuallyrequested by the host) and stores them in buffer 38. At a certain pointin time, the MSP sends a page having an index K to the host, and at thesame time reads page K+N from device 24 and caches it in buffer 38. Whendecoding page K, signal processing unit 48 in the MSP retrieves page Kand its neighbors (at least some of which were not yet requested by thehost at the given point in time) from buffer 38. This sort of pipelinedoperation enables unit 48 to perform functions such as cross-couplingestimation and cancellation, and possibly ECC decoding, based on page Kand its neighbors, even though some or all of the neighbors were not yetrequested by the host.

In some embodiments, when rearranging the data pages (e.g., at step 78of the method of FIG. 3), the MSP programs the data pages in ascendingorder of the word lines, but in reverse order of the data pages. Inother words, if a given block holds M data pages, then the MSP storesthe M^(th) data page in the first word line, and continues until storingthe first data page in the M^(th) word line. In many scenarios, thisprogramming order reduces the reading delay when performingcross-coupling interference cancellation.

In some embodiments, the data is initially stored in a Dynamic RandomAccess Memory (DRAM), and then copied into a small number of memoryplanes of a non-volatile memory, such as Flash memory. Such aconfiguration may be advantageous, for example, in Solid State Disk(SSD) systems, but can be applied in other applications, as well.Initial storage in DRAM is usually fast, e.g., because DRAMs often storedata at a large word width. After initial storage, the data can becopied to non-volatile memory using the methods described herein. Insome implementations, the initially-stored data can be copied intomultiple planes of the non-volatile memory using multi-plane writecommands.

Although the embodiments described herein mainly address datarearrangement in multi-plane memory devices, the principles of thepresent invention can also be used for other applications, such asapplications that store data in multiple dies or devices. For example,some data storage systems receive data streams from multiple sources andstore them in an interleaved manner. Using the disclosed methods, suchsystems may initially store the data streams in an interleaved fashion,and later copy them so that the data of each data stream is arrangedconsecutively. The rearranged format enables reading each data streamseparately at a higher speed.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. A method for data storage, comprising:performing, by a memory system including multiple memory arrays,operations including: initially storing a sequence of data pages in thememory arrays, such that successive data pages in the sequence arestored in alternation in a first number of memory arrays; rearrangingthe initially-stored data pages in the memory so as to store thesuccessive data pages in the sequence in a second number of the memoryarrays, which is less than the first number such that, readout of thesequence of data pages results in a given data page and a set of theneighboring data pages being stored together in a page buffer; andreading the rearranged data pages from the second number of the memoryarrays, and canceling the interference in the given data page using apredefined number of the neighboring data pages cached in the pagebuffer.
 2. The method according to claim 1, wherein reading therearranged data pages comprises reading a given data page and one ormore neighboring data pages from the second number of the memory arraysand canceling cross-coupling interference in the given data pageresponsively to the read neighboring data pages.
 3. The method accordingto claim 2, wherein reading the rearranged data pages comprises cachingthe read data pages in a cache memory such that both the given data pageand the neighboring data pages are cached in the cache memory at a giventime, and wherein canceling the cross-coupling interference comprisescanceling the interference using the cached given data page andneighboring data pages.
 4. The method according to claim 1, wherein thesecond number of the memory arrays comprises a single memory array. 5.The method according to claim 1, wherein the second number of the memoryarrays comprises two or more memory arrays, and wherein rearranging theinitially-stored data pages comprises storing the successive data pagesin alternation in the second number of the memory arrays.
 6. The methodaccording to claim 1, wherein rearranging the initially-stored datapages comprises rearranging the initially-stored data pages so as tostore the successive data pages in a third number of the memory arrays,which is smaller than the first number and larger than the secondnumber, and rearranging the data pages stored in the third number of thememory arrays so as to store the successive data pages in the secondnumber of the memory arrays.
 7. The method according to claim 1, whereinrearranging the initially-stored data pages comprises defining multiplegroups of the memory arrays, each group having the second number of thememory arrays, and storing respective subsets of the successive datapages in the groups.
 8. The method according to claim 7, wherein storingthe subsets in the groups comprises sending multi-plane write commands,each command specifying storage of the second number of the data pagesin each of the groups.
 9. The method according to claim 1, whereininitially storing the sequence comprises sending multi-plane writecommands, each command specifying storage of the first number of thedata pages in the first number of the memory arrays.
 10. The methodaccording to claim 1, wherein initially storing the sequence comprisessending an acknowledgement indication responsively to a successfulcompletion of initially-storing the sequence, irrespective ofrearrangement of the initially-stored data pages.
 11. The methodaccording to claim 1, wherein rearranging the initially-stored datapages comprises rearranging the data pages during an idle period inwhich no memory access operations are performed.
 12. The methodaccording to claim 1, wherein each of the memory arrays comprises aplurality of analog memory cells, wherein initially storing the sequencecomprises programming the memory cells in the first number of the memoryarrays using first programming parameters, and wherein rearranging thedata pages comprises programming the memory cells in the second numberof the memory arrays using second programming parameters, different fromthe first programming parameters.
 13. The method according to claim 12,wherein programming the memory cells using the first programmingparameters comprises programming the memory cells to assume a firstnumber of programming levels, and wherein programming the memory cellsusing the second programming parameters comprises programming the memorycells to assume a second number of programming levels, larger than thefirst number.
 14. The method according to claim 12, wherein programmingthe memory cells using the first programming parameters comprisesapplying to the memory cells a first sequence of programming pulseswhose amplitude increases incrementally by a first step size, andwherein programming the memory cells using the second programmingparameters comprises applying to the memory cells a second sequence ofprogramming pulses whose amplitude increases incrementally by a secondstep size, smaller than the first step size.
 15. The method according toclaim 1, wherein reading the rearranged data pages comprises: receivinga request from a host to retrieve one or more requested data pages inthe sequence; responsively to the request, reading from the secondnumber of the memory arrays one or more non-requested data pages in thesequence, which were not requested by the host, as well as the requesteddata pages; caching the read requested and non-requested data pages; andcanceling cross-coupling interference in at least one of the requesteddata pages using the cached requested and non-requested data pages. 16.The method according to claim 1, wherein initially storing the datapages comprises storing the data pages in a Dynamic Random Access Memory(DRAM), and wherein rearranging the initially-stored data pagescomprises copying the data pages from the DRAM to a non-volatile memory.17. The method according to claim 1, wherein the memory cells in thesecond number of the memory arrays are arranged along multiple wordlines, and wherein rearranging the initially-stored data pages comprisesstoring the data pages in an ascending order of the word lines and in areverse order of the sequence of data pages.
 18. A method for datastorage, comprising: initially storing, by a processor, a sequence ofdata pages in a memory comprising a plurality of memory arrays, suchthat successive data pages in the sequence are distributed across afirst number of memory arrays of the plurality of memory arrays;rearranging the initially-stored data pages in the memory so as todistribute the successive data pages in the sequence of data pages in asecond number of memory arrays of the plurality of memory arrays,wherein the second number of memory arrays is less than the first numberof memory arrays; and reading a selected rearranged data page from thesecond number of memory arrays, wherein the reading comprises performingan interference cancelling operation on the selected rearranged datapage dependent upon other rearranged data pages stored in the samememory array as the selected rearranged data page.
 19. The methodaccording to claim 18, wherein memory cells in the second number ofmemory array of the plurality of memory arrays are arranged alongmultiple word lines, and wherein rearranging the initially-stored datapages comprises storing the data pages in an ascending order of the wordlines and in a reverse order of the sequence of data pages.
 20. Themethod according to claim 18, wherein the reading the selectedrearranged data page comprises storing the rearranged data page furthercomprises storing together the selected rearranged data page, and a setof neighboring data pages from the other rearrange data pages in a pagebuffer.
 21. The method according to claim 20, wherein the performing aninterference cancelling operation comprises operating on data pagesstored in the page buffer.